1. Field of the Invention
The present invention relates to a rack and pinion type power steering apparatus characterized by a rack bush slidably supporting a rack shaft and an attaching structure of the rack bush.
2. Description of the Related Art
In a rack and pinion type steering apparatus, a structure is well known in which a rack shaft is slidably supported to a rack housing corresponding to a support portion of the rack shaft by a gear housing portion near one end of the rack shaft and a housing portion near the other end. The rack shaft is slidably supported in the support portions via a rack bush made of synthetic resin, rubber or the like.
Further, since the support of the rack shaft by the rack bush at the gear housing position near one end is an engaging position between a rack tooth of the rack shaft and a pinion, specific structural consideration is given to the bush and an attachment of the bush (refer, for example, to Japanese Patent Application Laid-open No. 3-193556 (patent document 1) (page 2, FIGS. 1 and 2) and Japanese Patent Application Laid-open No. 2002-274396 (patent document 2) (pages 3 to 4, FIGS. 1 to 3).
The support of a rack shaft 01 in a gear housing portion 04 described in the patent document 1 mentioned above is achieved by one rack bush 05 comprising an elastic member made of synthetic resin, rubber or the like as illustrated in FIGS. 7A and 7B. The support of the rack shaft 01 by the bush 05 is structured so as to support a back surface side of an engagement portion between a rack tooth 02 of the rack shaft 01 and a pinion 03, so as to press the surface toward the pinion 03. The rack bush 05 is provided with a plurality of projections 06 serving as a movement inhibiting means in an axial direction under an attached state, in an outer periphery of the bush 05. The projections 06 are provided with a structure engaging with a recess portion in an inner periphery of the gear housing portion 04 and a step portion.
Support of the rack shaft 01 in the gear housing portion (the rack housing) 04 described in the patent document 2 mentioned above is achieved by a cylindrical rack bush 05 constituted by an elastic member made of a synthetic resin or the like as illustrated in FIGS. 8A to 8C. The bush 05 is provided with a pair of pressing projections 06 in which a part of a cylindrical cross section is bulged to an inner surface side of the cylinder. The projection 06 is structured so to extend all the region in an axial direction of the bush 05, and is arranged to be approximately one half in a circumferential direction of the bush 05.
The rack bush 05 is structured such that one end surface of a cylindrical shape thereof is brought into contact with a step portion 041 in an inner periphery of the gear housing portion 04 so as to be positioned in an axial direction and to be press fit into a large-diameter portion 042 of the housing 04. A pair of pressing projections 06 is brought into contact with a back surface of the rack tooth 02 of the rack shaft 01, thereby pressing the rack shaft 01 in a direction of the pinion 03, that is, a direction of the engagement portion between the rack tooth 02 and the pinion 03. This secures the engagement between the rack tooth 02 and the pinion 03 with no play, whereby a rotating torque of the pinion 03 is smoothly transmitted to the rack shaft 01.
In this case, the rack bush (the elastic member) described in the patent document 1 is provided with a structure in which a projection in an outer peripheral portion engages with a recess portion or a step portion formed in an inner periphery (an inner surface) of a gear housing so as to inhibit the bush from moving. However, a relatively large deformation is generated in the bush during insertion into the inner peripheral portion of the gear housing because of the structure of the rack bush. Accordingly, if the bush is not made of a soft material, the bush can not be inserted. Further, it is considered that a slit may be provided in the bush for the purpose of making it easier to insert the bush, however, in this case, the bush tends to be disconnected.
Further, the rack bush (the elastic member) described in the patent document 2 can be easily pressure inserted from an axial direction during attaching. However, since no positioning means acting as a rotation prevention means in a rotating direction is provided, it is difficult to achieve an accurate positioning in the rotating direction of the pressure insertion of the rack bush. Since displacement in the rotating direction tends to be generated together with a passage of time after being attached in accordance with the pressure insertion, it is difficult to keep a proper engagement between the rack tooth of the rack shaft and the pinion, and it is difficult to effectively inhibit a gear engaging sound from being generated.
Under these circumstances, it is desirable to provide the rack and pinion type power steering apparatus mentioned above in which an improvement relating to the structure of the rack bush and the support structure by the bush is provided in the slidable support of the rack shaft by the rack bush at the gear housing position corresponding to the engagement portion between the rack tooth of the rack shaft and the pinion. This effectively inhibits the play due to the oscillation of the rack shaft and makes the engagement between the rack tooth of the rack shaft and the pinion proper, whereby it is possible to effectively inhibit the gear engaging sound generated from the engagement portion.